target-dependent .gdbinit

[Felix Lee]

So I'm thinking I want gdb to read ~/.gdbinit-$target after
~/.gdbinit.  Does that seem like a sensible thing to do?

I'm wary of making target a special case.  My feeling is I'd
want a single .gdbinit file that has some form of #if for
target or host or gdb version or other conditions.  But I'm
not sure that's all that useful.  The added complexity
doesn't seem to be worth it.

But target is already a special case sometimes, because
there's .vxgdbinit and such.

Also, behavior like this could be easily synthesized with a
front-end script that ran gdb with the right -x options.
The rationale for putting it into gdb: it's standardization.
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